1. Field of the Invention
The invention relates to a low noise amplifier, and more particularly, to a dynamically adjusting a bias voltage low noise amplifier when switching gain modes to improve linearity.
2. Description of the Prior Art
A low noise amplifier is the first amplifying circuit in the front-end receiver of the wireless communication system. Normally, any noise that exists in the transmitting channel, like air, will always contaminate a transmitting signal. When the receiver receives the transmitting signal, the low noise amplifier provides a gain to amplify the received signal to an acceptable power, but also to attenuate the noise in the transmitting signal simultaneously. Furthermore, regarding a receiver being located closest to a base station of the wireless communication system, the gain of the low noise amplifier should be adjust to a rather low gain to prevent the receiver from entering a saturated situation. The saturated situation can result because the amplitude of the transmitting signal is higher corresponding in relation to the closeness of the base station; in other words, the power of the received signal is higher.
Therefore, a rather low gain of the low noise amplifier will guarantee a normal operation of the receiver in the case where the receiver and the base station are close to each other. Therefore, most of the low noise amplifiers are capable to adjusting the gain according to the power of the received signal. Additionally, certain other low noise amplifiers are capable of switching between a high gain operating mode and a low gain operating mode as needed based on said distance conditions.
In reality, there are two problems that will result due to the above-mentioned low noise amplifier strategies. Firstly, when the low noise amplifier is switching from the high gain operating mode into the low gain operating mode, in other words, that is when the amplitude of the received signal is high, the low noise amplifier is not able to provide an acceptable linearity to the received signal simultaneously and this will consequently cause the received signal to be different from the transmitting signal. Secondly, when the low noise amplifier is switching between a high gain operating mode and a low gain operating mode, the signal path of the received signal will probably change the equivalent circuit of the low noise amplifier and the stable condition of the circuit of the low noise amplifier. Therefore, an oscillation may result.
Please refer to FIG. 1. FIG. 1 illustrates a diagram of a switching gain mode low noise amplifier 100 of the prior art. The configuration of the low noise amplifier 100 is a single-end-input differential-output configuration, wherein the input signal LNAIN at the single-end-input is transformed into a differential input signal. The differential input signal is then amplified by a two stage amplifier to generate a differential output signal; wherein the second stage 104 is an adjustable gain output stage. The control signal G0, G0B (i.e., an inverse signal of G0), G2, G2B (i.e., an inverse signal of G2) control the status of conducting/non-conducting of the transistor in the second stage 104 to adjust the gain of the low noise amplifier 100. Accordingly, the low noise amplifier is capable to switching between the high gain operating mode and the low gain operating mode. However, the switching gain low noise amplifier of the prior art has not overcome the above-mentioned linearity problems, furthermore, the embedded transformer 102 increases the chip area of the prior art receiver. Additionally, the embedded transformer 102 also causes degradation to the input signal.
Please refer to FIG. 2. FIG. 2 illustrates a diagram of another switching gain mode low noise amplifier 200 of the prior art. The configuration of the low noise amplifier 200 is a single-input/single-output configuration. The low noise amplifier 200 includes a first input stage 202, a second gain stage 204, and a switching circuit 206. According to FIG. 2, the signal path of the high gain operating mode is different from the signal path of the low gain operating mode in the low noise amplifier 200 in which the oscillation phenomenon may occur. On the other hand, when the low noise amplifier 200 is switching between the high gain operating mode and the low gain operating mode, the condition of input matching of the input impedance is easily changed and this will cause the input signal to be reflected from the input of the low noise amplifier 200. Please refer to the U.S. Pat. No. 6,175,274, for the corresponding technique.